Am I wrong ?

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AdamR

AdamR

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I was looking at the Mopar Performance crate motors and something didnt seem right.

First I looked at the 9:1 Magnum block, Mopar says it has a 1.67 Compression height So I do alittle math. 1.79 (half of stroke) + 6.123 (rod length) + 1.67(compression height) = 9.583. No subtract that from the deck Height 9.56 (on the short blocks) = -0.023 which means the pistons stick .023" out of the hole. Now swinging over to the compression calculator at http://www.straightline-perf.com/crcalculator.shtml and using a comman
fel pro gasket with a 4.180 bore and a .039 compressed thickness and a 62cc head (magnum) I get 12.28:1

Now this is a Magnum short block, its set up for magnum roller cam and pushrod oiling so one would assume that the 10:1 rating would be for Magnum heads. But to actually get the 10:1 compression you would need to add 16cc worth of volume between the piston and head wich means either a .110" thick gasket wich no one makes and probably wouldnt last long or a 78cc head wich would be wich is average size for 340 or 360s.

I wonder how many people out there are wondering why they need 114 oct to run a 10:1 street motor ?
 
I bought the high compression MP short block and there were some misprints. Mine is not a magnum but I think they are the same. The pistons are .011 out of the block. With a .038 FelPro gasket that is .027 piston to head clearance. I've twisted this motor to 7500 without problems.
 
Something else to think of Adam, they are bolting parts together, not blueprinting. I'd be VERY impressed if they have the correct deck height on it. (take guitar's reading of .011 positive deck on the pistons, when your calculations say it should be .023)
 
If I went with one it would have to come a part. posative deck height and closed chambered heads make bad noises.
 
Adam, For what it's worth I went throught the same calculations as you and came up with the same results that the pistons would stick out of the block. Everything I have read in the mopar small block books suggests that the deck height is notoriously wrong, on the tall side. When I took mine apart and measured everything that was true.

I just got my block and crank back from the machine shop and they had to take 0.030" off the block to square the decks and that put the KB107 flat tops right at zero deck. The KB's have a 1.675 compression height.

With the same Fel-Pro head gasket and magnum heads that puts me at 10.8:1 compression. I had wanted the machine shop to keep the pistons at about 0.010-0.020 below the deck to keep the compression at a more pump gas friendly level but with the twist in the decks it was not possible.

The machine shop I used is highly regarded for race engines and the owner feels that with the cam I am using, the perfect quench clearance at zero deck and with the magnum heads that I will have less detonation issues that than with the LA open chamber heads and 9.x:1 compression.

I hope to have the engine all assemblied and in with in the next month or so and will report on how it goes. If it turns out that I need to lower the compression I have the option of using the LA heads, going with a Comtec head gasket (can get in any thickness up to .125) or what I would realy like to do is put on a set of edlebrock Aluminum heads.
 
Only if they stick out further than the compressed thickness of the head gasket. Then it's more than a slight problem, the pistons will crash into the head.
 
but for a good quench distance you want about .040" so that means zero deck and a .040 gasket or a gasket that is .040" thicker then the piston is sticking ot of the bore.
 
I'd take a crate motor apart in any case, just to make sure it's done properly. Quality control issues have also plagued the crate engines.
 
moper said:
I'd take a crate motor apart in any case, just to make sure it's done properly. Quality control issues have also plagued the crate engines.
Click to expand...

I've heard of missing bearings and blocks with 2 differant over bores on each side of the block.

I keep forgetting I have a 318 Magnum block I can play with also.
 
The parts are worth the price of admission, it's the assembly and machining they make the $$ on. In most cases, it's thing left untorqued, or timing sets in wrong. I have heard of several that were noisey(knocking) the first time they were fired. as a performance part, they are not warranteed..perfect scenario for problems. A typical performance rebuilt engine will cost roughly $3-500 more than a crate, but you can get better parts, and better machining for the $$. Or, you buy the crate, pay a shop to tear it down, and check, and reassemble (usually around 2-300 plus gaskets) and you KNOW it's right. Then you can have the shop degree the cam, set compression, etc.
 
FWIW, I have a complete 318 Magnum in the garage.The pistons are flat top and are about 0.040 in the hole.
 
How much work do you think it would take to get a 318 Mag in the mid 12s ?
.040" dowm with a Comtec .020" gasket for the 318 bore and the 62cc would be 9.8:1 wich is perfect. But then Im giving up 22 cubes from the my 340 or 42 from going to a 360
I could get a nice HYd roller cam ground. But is the gain in compression worth the loss in cubes.
 
heard about a former employe where i work who bought a brand new chevy LS7 engine and took it apart the first thing he did and it wasnt square or round at any place.. every measure taken on that engine was far of from anything looking like even a good standard engine but that was many years ago its sad if crate engines are still that bad...
 
MP changed the supplier of their crate engines a few years ago and I don't believe they have the quality issues they had on some of the engines. I know three people that have purchased them in the past couple of years and they dropped them and been running troble free since. Also, even though the weren't warranteed MP did stand behind them when folks had problems.
 
Well, it's good to hear MP is standing behind their engines.

Once upon a time I was going to purchase a 380hp 360. I didn't have a car yet, but thought that would be a great street engine.

Of course, that was before I bought my car, which came with a fully machined block, crank, heads, blah blah blah....
 
AdamR said:
How much work do you think it would take to get a 318 Mag in the mid 12s ?
.040" dowm with a Comtec .020" gasket for the 318 bore and the 62cc would be 9.8:1 wich is perfect. But then Im giving up 22 cubes from the my 340 or 42 from going to a 360
I could get a nice HYd roller cam ground. But is the gain in compression worth the loss in cubes.
Click to expand...

It's as hard (or as easy) as a non magnum. Just a matter or spending money, and matching components. IMO, the hyd rollers are overrated, when the application moves beyond a street engine. The heads are the best parts about those engines. The shortblock is just run of the mill.
 
Guys, you aren't paying attention. The short block I have has the pistons .011 out of the hole. I use a 1008 FelPro head gasket at .0385 compressed with small chamber W5 heads milled .050. You want to talk about a closed combustion chamber? It's 47 cc's. I twist this thing to 7500 and it has never touched a head. You can see where it is though on the piston top as well as the valves. The edge of the intake valve is actually below the deck surface of the head so it's even closer to the piston than the head is.

I had a talk with David Crume of Crume Racing Engines (CRE) after I measured everything, (he does what machine work I can't do for me and has never steered me wrong nor have I ever gotten anything out of his shop that wasn't exactly what I asked for) and he said he likes to set the quench up at .038 but you can get by with as little as .025 with a steel connecting rod and new bearings. I figure I have a little cushion then at .0275 :D

BTW compression ratio is 12.7:1
 
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